diff --git a/include/mjx/internal/state.cpp b/include/mjx/internal/state.cpp
index 498ae9cde..9a5b7d365 100644
--- a/include/mjx/internal/state.cpp
+++ b/include/mjx/internal/state.cpp
@@ -3,8 +3,6 @@
 #include <google/protobuf/util/json_util.h>
 #include <google/protobuf/util/message_differencer.h>
 
-#include <queue>
-
 #include "mjx/internal/utils.h"
 
 namespace mjx::internal {
@@ -260,134 +258,9 @@ mjxproto::State State::LoadJson(const std::string &json_str) {
 State::State(const std::string &json_str) : State(LoadJson(json_str)) {}
 
 State::State(const mjxproto::State &state) {
-  // Set player ids
-  state_.mutable_public_observation()->mutable_player_ids()->CopyFrom(
-      state.public_observation().player_ids());
-  // Set scores
-  state_.mutable_public_observation()->mutable_init_score()->CopyFrom(
-      state.public_observation().init_score());
-  curr_score_.CopyFrom(state.public_observation().init_score());
-  // Set walls
-  auto wall_tiles = std::vector<Tile>();
-  for (auto tile_id : state.hidden_state().wall())
-    wall_tiles.emplace_back(Tile(tile_id));
-  wall_ = Wall(round(), wall_tiles);
-  state_.mutable_hidden_state()->mutable_wall()->CopyFrom(
-      state.hidden_state().wall());
-  // Set seed
-  state_.mutable_hidden_state()->set_game_seed(
-      state.hidden_state().game_seed());
-  // Set dora
-  state_.mutable_public_observation()->add_dora_indicators(
-      wall_.dora_indicators().front().Id());
-  state_.mutable_hidden_state()->add_ura_dora_indicators(
-      wall_.ura_dora_indicators().front().Id());
-  // Set init hands
-  for (int i = 0; i < 4; ++i) {
-    players_[i] =
-        Player{state_.public_observation().player_ids(i), AbsolutePos(i),
-               Hand(wall_.initial_hand_tiles(AbsolutePos(i)))};
-    state_.mutable_private_observations()->Add();
-    state_.mutable_private_observations(i)->set_who(i);
-    for (auto t : wall_.initial_hand_tiles(AbsolutePos(i))) {
-      state_.mutable_private_observations(i)
-          ->mutable_init_hand()
-          ->mutable_closed_tiles()
-          ->Add(t.Id());
-    }
-    // set game_id
-    state_.mutable_public_observation()->set_game_id(
-        state.public_observation().game_id());
-  }
-
-  // Initial draw from dealer
-  Draw(dealer());
-
-  // sync curr_hand
-  for (int i = 0; i < 4; ++i) SyncCurrHand(AbsolutePos(i));
-
-  // Update by events
-  std::queue<mjxproto::Action> actions;
-  int last_ron_target = -1;
-  int last_ron_target_tile = -1;
-  for (const auto &event : state.public_observation().events()) {
-    if (Any(event.type(),
-            {mjxproto::EVENT_TYPE_DISCARD, mjxproto::EVENT_TYPE_TSUMOGIRI,
-             mjxproto::EVENT_TYPE_ADDED_KAN})) {
-      last_ron_target = event.who();
-      last_ron_target_tile = event.tile();
-    }
-    if (event.type() == mjxproto::EVENT_TYPE_ABORTIVE_DRAW_THREE_RONS) {
-      assert(last_ron_target != -1);
-      assert(last_ron_target_tile != -1);
-      for (int i = 0; i < 4; ++i) {
-        if (i == last_ron_target) continue;
-        mjxproto::Action ron =
-            Action::CreateRon(AbsolutePos(i), Tile(last_ron_target_tile),
-                              state_.public_observation().game_id());
-        actions.push(ron);
-      }
-      continue;
-    }
-    std::optional<mjxproto::Action> action = Action::FromEvent(event);
-    if (action) actions.push(action.value());
-  }
-
-  while (state.public_observation().events_size() >
-         state_.public_observation().events_size()) {
-    auto observations = CreateObservations();
-    std::unordered_set<PlayerId> is_action_set;
-    std::vector<mjxproto::Action> action_candidates;
-
-    // set action from next_action
-    while (true) {
-      if (actions.empty()) break;
-      mjxproto::Action next_action = actions.front();
-      bool should_continue = false;
-      for (const auto &[player_id, obs] : observations) {
-        if (is_action_set.count(player_id)) continue;
-        std::vector<mjxproto::Action> legal_actions = obs.legal_actions();
-        bool has_next_action =
-            std::count_if(legal_actions.begin(), legal_actions.end(),
-                          [&next_action](const mjxproto::Action &x) {
-                            return Action::Equal(x, next_action);
-                          });
-        if (has_next_action) {
-          action_candidates.push_back(next_action);
-          is_action_set.insert(player_id);
-          actions.pop();
-          should_continue = true;
-          break;
-        }
-      }
-      if (!should_continue) break;
-    }
-
-    // set no actions
-    for (const auto &[player_id, obs] : observations) {
-      if (is_action_set.count(player_id)) continue;
-      std::vector<mjxproto::Action> legal_actions = obs.legal_actions();
-      auto itr = std::find_if(legal_actions.begin(), legal_actions.end(),
-                              [](const mjxproto::Action &x) {
-                                return x.type() == mjxproto::ACTION_TYPE_NO;
-                              });
-      Assert(itr != legal_actions.end(),
-             "Legal actions should have No Action.\nExpected:\n" +
-                 ProtoToJson(state) + "\nActual:\n" + ToJson());
-      auto action_no = *itr;
-      action_candidates.push_back(action_no);
-    }
-
-    Assert(action_candidates.size() == observations.size(),
-           "Expected:\n" + ProtoToJson(state) + "\nActual:\n" + ToJson() +
-               "action_candidates.size():\n" +
-               std::to_string(action_candidates.size()) +
-               "\nobservations.size():\n" +
-               std::to_string(observations.size()));
-
-    Update(std::move(action_candidates));
-  }
-
+  SetInitState(state, *this);
+  std::queue<mjxproto::Action> actions = EventsToActions(state);
+  UpdateByActions(state, actions, *this);
   Assert(google::protobuf::util::MessageDifferencer::Equals(state, proto()),
          "Expected:\n" + ProtoToJson(state) + "\nActual:\n" + ToJson());
 }
@@ -1752,4 +1625,160 @@ std::string State::ProtoToJson(const mjxproto::State &proto) {
   Assert(status.ok());
   return serialized;
 }
+
+std::vector<std::pair<mjxproto::Observation, mjxproto::Action>>
+State::GeneratePastDecisions(const mjxproto::State &proto) noexcept {
+  State st;
+  SetInitState(proto, st);
+  std::queue<mjxproto::Action> actions = EventsToActions(proto);
+  auto decisions = UpdateByActions(proto, actions, st);
+  Assert(google::protobuf::util::MessageDifferencer::Equals(proto, st.proto()),
+         "Expected:\n" + ProtoToJson(proto) + "\nActual:\n" + st.ToJson());
+  return decisions;
+}
+
+void State::SetInitState(const mjxproto::State &proto, State &state) {
+  // Set player ids
+  state.state_.mutable_public_observation()->mutable_player_ids()->CopyFrom(
+      proto.public_observation().player_ids());
+  // Set scores
+  state.state_.mutable_public_observation()->mutable_init_score()->CopyFrom(
+      proto.public_observation().init_score());
+  state.curr_score_.CopyFrom(proto.public_observation().init_score());
+  // Set walls
+  auto wall_tiles = std::vector<Tile>();
+  for (auto tile_id : proto.hidden_state().wall())
+    wall_tiles.emplace_back(Tile(tile_id));
+  state.wall_ = Wall(state.round(), wall_tiles);
+  state.state_.mutable_hidden_state()->mutable_wall()->CopyFrom(
+      proto.hidden_state().wall());
+  // Set seed
+  state.state_.mutable_hidden_state()->set_game_seed(
+      proto.hidden_state().game_seed());
+  // Set dora
+  state.state_.mutable_public_observation()->add_dora_indicators(
+      state.wall_.dora_indicators().front().Id());
+  state.state_.mutable_hidden_state()->add_ura_dora_indicators(
+      state.wall_.ura_dora_indicators().front().Id());
+  // Set init hands
+  for (int i = 0; i < 4; ++i) {
+    state.players_[i] =
+        Player{state.state_.public_observation().player_ids(i), AbsolutePos(i),
+               Hand(state.wall_.initial_hand_tiles(AbsolutePos(i)))};
+    state.state_.mutable_private_observations()->Add();
+    state.state_.mutable_private_observations(i)->set_who(i);
+    for (auto t : state.wall_.initial_hand_tiles(AbsolutePos(i))) {
+      state.state_.mutable_private_observations(i)
+          ->mutable_init_hand()
+          ->mutable_closed_tiles()
+          ->Add(t.Id());
+    }
+    // set game_id
+    state.state_.mutable_public_observation()->set_game_id(
+        proto.public_observation().game_id());
+  }
+
+  // Initial draw from dealer
+  state.Draw(state.dealer());
+
+  // sync curr_hand
+  for (int i = 0; i < 4; ++i) state.SyncCurrHand(AbsolutePos(i));
+}
+
+std::queue<mjxproto::Action> State::EventsToActions(
+    const mjxproto::State &proto) {
+  std::queue<mjxproto::Action> actions;
+  int last_ron_target = -1;
+  int last_ron_target_tile = -1;
+  for (const auto &event : proto.public_observation().events()) {
+    if (Any(event.type(),
+            {mjxproto::EVENT_TYPE_DISCARD, mjxproto::EVENT_TYPE_TSUMOGIRI,
+             mjxproto::EVENT_TYPE_ADDED_KAN})) {
+      last_ron_target = event.who();
+      last_ron_target_tile = event.tile();
+    }
+    if (event.type() == mjxproto::EVENT_TYPE_ABORTIVE_DRAW_THREE_RONS) {
+      assert(last_ron_target != -1);
+      assert(last_ron_target_tile != -1);
+      for (int i = 0; i < 4; ++i) {
+        if (i == last_ron_target) continue;
+        mjxproto::Action ron =
+            Action::CreateRon(AbsolutePos(i), Tile(last_ron_target_tile),
+                              proto.public_observation().game_id());
+        actions.push(ron);
+      }
+      continue;
+    }
+    std::optional<mjxproto::Action> action = Action::FromEvent(event);
+    if (action) actions.push(action.value());
+  }
+  return actions;
+}
+
+std::vector<std::pair<mjxproto::Observation, mjxproto::Action>>
+State::UpdateByActions(const mjxproto::State &proto,
+                       std::queue<mjxproto::Action> &actions, State &state) {
+  std::vector<std::pair<mjxproto::Observation, mjxproto::Action>> hist;
+
+  while (proto.public_observation().events_size() >
+         state.state_.public_observation().events_size()) {
+    auto observations = state.CreateObservations();
+    std::unordered_map<PlayerId, mjxproto::Action> action_candidates;
+
+    // set action from next_action
+    while (true) {
+      if (actions.empty()) break;
+      mjxproto::Action next_action = actions.front();
+      bool should_continue = false;
+      for (const auto &[player_id, obs] : observations) {
+        if (action_candidates.count(player_id)) continue;
+        std::vector<mjxproto::Action> legal_actions = obs.legal_actions();
+        bool has_next_action =
+            std::count_if(legal_actions.begin(), legal_actions.end(),
+                          [&next_action](const mjxproto::Action &x) {
+                            return Action::Equal(x, next_action);
+                          });
+        if (has_next_action) {
+          action_candidates[player_id] = next_action;
+          actions.pop();
+          should_continue = true;
+          break;
+        }
+      }
+      if (!should_continue) break;
+    }
+
+    // set no actions
+    for (const auto &[player_id, obs] : observations) {
+      if (action_candidates.count(player_id)) continue;
+      std::vector<mjxproto::Action> legal_actions = obs.legal_actions();
+      auto itr = std::find_if(legal_actions.begin(), legal_actions.end(),
+                              [](const mjxproto::Action &x) {
+                                return x.type() == mjxproto::ACTION_TYPE_NO;
+                              });
+      Assert(itr != legal_actions.end(),
+             "Legal actions should have No Action.\nExpected:\n" +
+                 ProtoToJson(proto) + "\nActual:\n" + state.ToJson());
+      auto action_no = *itr;
+      action_candidates[player_id] = action_no;
+    }
+
+    Assert(action_candidates.size() == observations.size(),
+           "Expected:\n" + ProtoToJson(proto) + "\nActual:\n" + state.ToJson() +
+               "action_candidates.size():\n" +
+               std::to_string(action_candidates.size()) +
+               "\nobservations.size():\n" +
+               std::to_string(observations.size()));
+
+    std::vector<mjxproto::Action> action_vec;
+    for (const auto &[player_id, obs] : observations) {
+      auto action = action_candidates[player_id];
+      hist.emplace_back(obs.proto(), action);
+      action_vec.push_back(action);
+    }
+    state.Update(std::move(action_vec));
+  }
+
+  return hist;
+}
 }  // namespace mjx::internal
diff --git a/include/mjx/internal/state.h b/include/mjx/internal/state.h
index 2b41fd84d..d7b20f776 100644
--- a/include/mjx/internal/state.h
+++ b/include/mjx/internal/state.h
@@ -5,6 +5,7 @@
 #include <boost/uuid/uuid.hpp>
 #include <boost/uuid/uuid_generators.hpp>
 #include <boost/uuid/uuid_io.hpp>
+#include <queue>
 #include <random>
 #include <string>
 #include <utility>
@@ -52,6 +53,8 @@ class State {
   GameResult result() const;
   State::ScoreInfo Next() const;
   mjxproto::Observation observation(const PlayerId& player_id) const;
+  static std::vector<std::pair<mjxproto::Observation, mjxproto::Action>>
+  GeneratePastDecisions(const mjxproto::State& proto) noexcept;
 
   static std::vector<PlayerId> ShufflePlayerIds(
       std::uint64_t game_seed, const std::vector<PlayerId>& player_ids);
@@ -163,6 +166,17 @@ class State {
 
   // protoのcurr_handを同期する。
   void SyncCurrHand(AbsolutePos who);
+
+  // protobufから初期状態（親のツモの直後）を抽出して、stateへセットする
+  static void SetInitState(const mjxproto::State& proto, State& state);
+  // protoのEvent系列で見えているイベントをAction系列へ変換して返す（Noは含まない。三家和了はロンが３つ連なる）
+  static std::queue<mjxproto::Action> EventsToActions(
+      const mjxproto::State& proto);
+  // stateがprotoと同じにものになるように、actionsからactionをpopしながらstateを更新する（actionsにはNoがないので、それらを補完する）
+  // 結果として現れたObservation, Actionのペアが返される
+  static std::vector<std::pair<mjxproto::Observation, mjxproto::Action>>
+  UpdateByActions(const mjxproto::State& proto,
+                  std::queue<mjxproto::Action>& actions, State& state);
 };
 }  // namespace mjx::internal
 
diff --git a/include/mjx/state.cpp b/include/mjx/state.cpp
index 15cfcc3fb..5c64d5ec3 100644
--- a/include/mjx/state.cpp
+++ b/include/mjx/state.cpp
@@ -5,6 +5,8 @@
 
 #include <utility>
 
+#include "mjx/internal/state.h"
+
 namespace mjx {
 mjx::State::State(mjxproto::State proto) : proto_(std::move(proto)) {}
 
@@ -31,4 +33,14 @@ bool State::operator==(const State& other) const noexcept {
 bool State::operator!=(const State& other) const noexcept {
   return !(*this == other);
 }
+
+std::vector<std::pair<Observation, Action>> State::past_decisions()
+    const noexcept {
+  std::vector<std::pair<Observation, Action>> decisions;
+  auto proto_decisions = internal::State::GeneratePastDecisions(proto());
+  for (const auto& [obs, action] : proto_decisions) {
+    decisions.emplace_back(Observation(obs), Action(action));
+  }
+  return decisions;
+}
 }  // namespace mjx
diff --git a/include/mjx/state.h b/include/mjx/state.h
index 04427bd8e..77e1b11b7 100644
--- a/include/mjx/state.h
+++ b/include/mjx/state.h
@@ -1,7 +1,9 @@
 #ifndef MJX_PROJECT_STATE_H
 #define MJX_PROJECT_STATE_H
 
+#include "mjx/action.h"
 #include "mjx/internal/mjx.grpc.pb.h"
+#include "mjx/observation.h"
 
 namespace mjx {
 using PlayerId = std::string;  // identical over different games
@@ -19,6 +21,7 @@ class State {
 
   // accessors
   const mjxproto::State& proto() const noexcept;
+  std::vector<std::pair<Observation, Action>> past_decisions() const noexcept;
 
  private:
   mjxproto::State proto_{};
diff --git a/tests_cpp/internal_state_test.cpp b/tests_cpp/internal_state_test.cpp
index e35a969d8..1b19667e3 100644
--- a/tests_cpp/internal_state_test.cpp
+++ b/tests_cpp/internal_state_test.cpp
@@ -1145,3 +1145,19 @@ TEST(internal_state, GameId) {
   EXPECT_EQ(state1.proto().public_observation().game_id(),
             state3.proto().public_observation().game_id());
 }
+
+TEST(internal_state, GeneratePastDecisions) {
+  auto json = GetLastJsonLine("upd-aft-ron3.json");
+  State state(json);
+  auto past_decisions = state.GeneratePastDecisions(state.proto());
+  // for (const auto& [obs, action]: GeneratePastDecisions) {
+  //   std::cerr << Observation(obs).ToJson() << "\t" <<
+  //   Action::ProtoToJson(action) << std::endl;
+  // }
+  EXPECT_EQ(std::count_if(past_decisions.begin(), past_decisions.end(),
+                          [](const auto &x) {
+                            mjxproto::Action action = x.second;
+                            return action.type() == mjxproto::ACTION_TYPE_RON;
+                          }),
+            3);
+}
diff --git a/tests_cpp/state_test.cpp b/tests_cpp/state_test.cpp
index 9bc5bf7ec..6cb1e1b27 100644
--- a/tests_cpp/state_test.cpp
+++ b/tests_cpp/state_test.cpp
@@ -30,3 +30,10 @@ TEST(state, op) {
   EXPECT_EQ(state1, state2);
   EXPECT_NE(state1, mjx::State());
 }
+
+TEST(state, past_decisions) {
+  auto state = mjx::State(sample_json);
+  auto past_decisions = state.past_decisions();
+  EXPECT_EQ(past_decisions.size(),
+            87);  // TODO: 87が正しい値かどうかは確認していない
+}